1. Field of the Invention
The present invention relates to a high-frequency component that can be used in a plurality of different mobile communication systems.
2. Description of the Related Art
A mobile communication apparatus that can operate in a plurality of communication systems using different frequency bands has been provided. Examples of the communication systems include GSM 1800 (DCS 1800) using a 1800 MHz band, GSM 1900 (PCS 1900) using a 1900 MHz band, GSM 850 using a 850 MHz band, and GSM 900 (EGSM 900) using a 900 MHz band.
Such a mobile communication apparatus includes a front end unit that performs demultiplexing and multiplexing of transmission/reception signals and has multiband functionality, such as quad-band functionality to support four communication systems, triple-band functionality to support three communication systems, or dual-band functionality to support two communication systems (see, e.g., Japanese Unexamined Patent Application Publication No. 2002-94410).
Typically, a front end unit of such a multiband mobile communication apparatus is defined by a module, and includes a diplexer connected to an antenna port and a plurality of switching circuits connected downstream of the diplexer.
For example, in a front end unit of a dual-band mobile communication apparatus, a diplexer performs demultiplexing and multiplexing of low-frequency transmission/reception signals (e.g., EGSM (GSM 900 or GSM 850) transmission/reception signals) and high-frequency transmission/reception signals (e.g., DCS or PCS transmission/reception signals). Then, in a low-frequency circuit downstream of the diplexer, an EGSM switching circuit performs switching between EGSM transmission and reception signals. Likewise, in a high-frequency circuit downstream of the diplexer, a switching circuit performs switching between DCS (PCS) transmission and reception signals.
In a front end unit of a triple-band or quad-band mobile communication apparatus, for example, additional switching circuits are provided downstream of the switching circuits described above, so that switching between GSM 850 and GSM 900 reception signals and switching between DCS and PCS reception signals are performed.
In the front end units described above, a reception path for each communication system may be provided with an unbalanced-input/balanced-output SAW filter that passes only a reception signal band to eliminate unwanted frequency signals and amplifies a reception signal. The SAW filter prevents signals from propagating from a transmission path to the reception path, and thus prevents problems in a circuit in the reception path.
In a typical circuit configuration of the diplexer, a high-frequency filter and a low-frequency filter are connected in parallel with an antenna port.
The high-frequency filter may be defined by a plurality of capacitors cascade-connected to the antenna port and a series resonant circuit having one end connected between the capacitors and the other end grounded. An impedance of each of these elements of the high-frequency filter is set such that a low-frequency transmission/reception signal is attenuated and a high-frequency transmission/reception signal is allowed to pass through.
To steepen a cutoff characteristic curve at an attenuation pole that prevents the low-frequency transmission/reception signal from reaching high frequencies, an impedance of each element of the series resonant circuit in the high-frequency filter is set such that the frequency of the attenuation pole is substantially equal to a trap frequency of the low-frequency transmission/reception signal (e.g., a resonant frequency substantially equal to a standard center frequency in the low-frequency communication system).
On the other hand, the low-frequency filter includes a parallel resonant circuit defined by a line connected to the antenna port and a capacitor connected in parallel with the line. An impedance of each of elements of the low-frequency filter is set such that a high-frequency transmission/reception signal is attenuated and a low-frequency transmission/reception signal is allowed to pass through.
To steepen a transmission characteristic curve at an attenuation pole that prevents the high-frequency transmission/reception signal from reaching low frequencies, an impedance of each element of the parallel resonant circuit in the low-frequency filter is set such that the frequency of the attenuation pole is substantially equal to a trap frequency of the high-frequency transmission/reception signal (e.g., a resonant frequency substantially equal to a standard center frequency in the communication system).
When a SAW filter is provided in a reception path in each communication system, since the impedance of the SAW filter is less than 50Ω at frequencies outside its passband, impedance match cannot be achieved in a connecting portion of the SAW filter. Thus, to achieve impedance matching, it is necessary to provide a phase adjusting circuit in the front end unit and to provide a multistage filter in the diplexer. This is disadvantageous because the circuit configuration becomes complex, the number of parts increases, and the size of the module increases.
If a SAW filter is provided in a reception path for a high-frequency or low-frequency communication system without performing impedance matching, transmission characteristics in a frequency band of one communication system and cutoff characteristics in a frequency band of the other communication system will be deteriorated.